Help with winterizing!

I have a few question here. I am trying to help out a neighbor with winterizing his system. We will go in on a rental so makes it pretty cheap to do it. Ive done mine for years and installed when new, a hook up to blow out. My neighbors system doesnt have any connection on or around his backflow to hook in to. The backblow doesnt have any unions on it either so he can not unhook it and bring it inside like i thought was best to do. It is 3/4 copper coming from the house and into the ground where it switches to 1" pvc. I guess my question is does you think its ok to cut into each copper riser and either install unions or the easiest would be two 3/4" shark bites and maker an adapter to hook the air line to. Any advice would be appreciated

Thanks for the reply. I know there is a drain on the ball valve in the basement. If I understand your idea correctly, I would like to not make a hookup for the air line in the basement as i would need the line ran through the basement through a window. For easy access was thinking about making some sort of blow out adapter on the system side of the back flow. When its time to blow out, turn off the valve inside and at backflow, then winterize. Ideally Id like to bring it inside for the winter, so may need shark bite couplers or solder in unions?